(a) Field of the Invention
The present invention relates to an objective lens system for endoscopes using optical fiber bundles or relay lenses as an image transmission optical system, and more specifically to an objective lens system having favourably corrected distortion.
(b) Description of the Prior Art
As the conventional objective lens systems for endoscopes of the retrofocus type as shown in FIG. 1, there has already been known, for example, the one disclosed by Japanese published unexamined patent application No. 121547/74.
This objective lens system for endoscopes of retrofocus type comprises a lens group I having negative refractive power and a rear lens group II having positive refractive power which are arranged on the object side and image side respectively with a stop S interposed therebetween. This objective lens system is so designed as to obtain a wide angle by strongly refracting the principal ray P with the negative lens group I arranged before the stop S. Further, the positive lens group II arranged after the stop S functions to make the principal ray P incident on the image surface in parallel with the optical axis.
The objective lens system is so designed as to minimize loss of light in the image guide G by making the principal ray P emerging from the objective lens incident perpendicularly on the end surface of the image guide G.
When the objective lens system of this type is to be applied to an endoscope equipped with relay lenses, the principal ray P is made perpendicular to the image surface O' as shown in FIG. 2 to minimize loss of rays in the relay lenses R.
The conventional objective lens system for endoscopes of retrofocus type satisfies two requirements for an objective lens system for endoscopes, i.e., a wide angle and perpendicular incidence of the principal ray on the image surface. However, there still remains a defect that negative distortion is remarkable in the objective lens system for endoscopes.
In the objective lens system for endoscopes shown in FIG. 1, for example, distortion is -21% at .omega.=37.degree. (2.omega.=angle of view). In addition, negative distortion is remarkable in the other conventional objective lens systems of retrofocus type as is seen from the relationship between angle of view and distortion listed in Table 1:
TABLE 1 ______________________________________ .omega. 20.degree. 30.degree. 40.degree. 50.degree. 60.degree. Distortion -6% -13.5% -23% -36% -50% ______________________________________
In order to correct the negative distortion, it is contrived to arrange an aspherical surface in the objective lens system. As an example of objective lens systems having distortion nd other aberrations corrected with an aspherical surface, there have been known the one disclosed by Japanese published unexamined patent application No. 173810/82. However, distortion is not corrected sufficiently in this objective lens system though it has a narrow angle of view (2.omega.) of 56.degree..